1. Field of the Invention
This invention relates to a DC arc furnace which is utilized in manufacturing steel principally from scraps as raw materials. It relates more particularly to a cooling structure for a bottom electrode provided in the bottom of the DC arc furnace.
2. Description of the Prior Art
The bottom electrode of the DC arc furnace utilized, for example, in manufacturing steel is in contact with molten metal at a temperature reaching 1600.degree..about.1800.degree. C. and is additionally heated to a high temperature by the joule's heat generated by the high density electric current flowing through the electrode. Accordingly, the bottom electrode is rapidly wasted. Furthermore, there is a danger of leaking out of the molten metal resulting from the excessively melted bottom electrode and so the cooling of the bottom electrode is regarded as important. As the means of cooling this bottom electrode, there is following one. Namely, the bottom electrode is made in the form of a rod with its lower portion extruding downwards through the bottom of the furnace and a water-cooled jacket is additionally mounted around the extruding portion of the electrode. The lower portion of the bottom electrode is cooled by supplying cooling water to the water-cooled jacket.
In the means constructed as mentioned above, the heat in the bottom electrode conducts only to such portion of the cooling water that flows by along the internal surface of the wall of the cooling jacket. As the result, the cooling efficiency per unit quantity of the cooling water is low. Consequently, a large amount of cooling water is necessary in order to cool sufficiently the bottom electrode.
Furthermore, in the means of the above mentioned construction, the heat conduction between the lower portion of the bottom electrode and the water-cooled jacket becomes poor and the cooling efficiency is lowered the more if there is an air gap between the lower portion of the bottom electrode and the water-cooled jacket. It is therefore necessary to make the both members in contact over a wide ares to improve the heat conduction. However, the bottom electrode expands thermally in its radial direction on account of the heat generated by the operation of the furnace. Moreover, the amount of the expansion of the bottom electrode varies depending on if the expanding portion is close to or away from the bottom of the furnace since the temperature of the portion is different according to its location. Accordingly, it is very difficult to make the water-cooled jacket contact homogeneously with the lower portion of the bottom electrode over the above mentioned wide area.